Two limit cases of twisted hBN bilayers and their excitonic response

TL;DR

This study analyzes the excitonic optical response of two extreme twisted hBN bilayers, AB and AA', using a novel computational approach, revealing distinct excitonic features and predicting the conductivity of the less-studied AB configuration.

Contribution

It introduces a simplified method to solve the Bethe-Salpeter equation for bilayers, enabling detailed excitonic response predictions for AB and AA' hBN bilayers.

Findings

Good agreement with existing ab initio results for AA' bilayer

Prediction of excitonic conductivity for AB bilayer, highlighting a specific resonance

Distinct excitonic behaviors between AB and AA' configurations

Abstract

In this paper we discuss the optical response due to the excitonic effect of two types of hBN bilayers: AB and AA'. Understanding the properties of these bilayers is of great utility to the study of twisted bilayers at arbitrary angles, since these two configurations correspond to the limit cases of 0 and 60 degree rotation. To obtain the excitonic response we present a method to solve a four-band Bethe-Salpeter equation, by casting it into a 1D problem, thus greatly reducing the numerical burden of the calculation when compared with strictly 2D methods. We find results in good agreement with ab initio calculations already published in the literature for the AA' bilayer, and predict the excitonic conductivity of the AB bilayer, which remains largely unstudied. The main difference in the conductivity of these two types of bilayers is the appearance of a small, yet well resolved, resonance between two larger ones in the AB configuration. This resonance is due to a mainly interlayer exciton, and is absent in the AA' bilayer. Also, the conductivity of the AB bilayer is due to both intralayer and interlayer excitons and is dominated by p-states, while intralayer states are the relevant ones for the AA' configuration, like in a monolayer. The effect of introducing a bias in the AA' bilayer is also discussed.